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Abstract:

The invention relates to a method for operating a compressor unit, in
particular for underwater operation. The aim of the invention is to
minimize the risk of gas hydrate formation during the compression
process. To reduce said risk, according to the method components of the
compressor unit are supplied with anti-freeze and/or anti-freeze is
injected into the flow path of the conveyed medium to be compressed. The
invention also discloses a compressor unit, which operates according to
said method.

Claims:

1-12. (canceled)

13. A method for compression of natural gas via a compressor,
comprising:operating the compressor under water; andapplying antifreeze
to components of the compressor unit.

14. The method as claimed in claim 13, wherein the antifreeze is injected
directly into the compressor of the compressor unit.

15. The method as claimed in claim 13, wherein the antifreeze is injected
directly into an intake connecting stub for the natural gas.

16. The method as claimed in claim 15, wherein the antifreeze is applied
to bearings or to a motor.

17. The method as claimed in claim 16, wherein antifreeze is injected into
an overflow between two compressor stages of the compressor of the
compressor unit.

18. The method as claimed in claim 17, wherein the pumping medium is
natural gas.

19. The method as claimed in claim 18, wherein the antifreeze is methyl
ethylene glycol.

20. The method as claimed in claim 19, wherein the antifreeze is injected
before the compressor unit is started.

21. The method as claimed in claim 20, wherein the antifreeze is added
only before the compressor unit is started.

22. The method as claimed in claim 19, wherein the antifreeze is added
before the compressor unit is stopped.

23. The method as claimed in claim 22, wherein the antifreeze is added
only before the compressor unit is started and before it is stopped.

24. A method for compression of natural gas via an underwater compressor,
comprising:operating the compressor under water; andinjecting antifreeze
into the flow path of the natural gas to be compressed.

25. The method as claimed in claim 24, wherein the antifreeze is injected
directly into the compressor of the compressor unit.

26. The method as claimed in claim 24, wherein the antifreeze is injected
directly into an intake connecting stub for the natural gas.

27. The method as claimed in claim 26, wherein the antifreeze is applied
to bearings or to a motor.

28. The method as claimed in claim 27, wherein antifreeze is injected into
an overflow between two compressor stages of the compressor of the
compressor unit.

29. The method as claimed in claim 28, wherein the pumping medium is
natural gas.

30. The method as claimed in claim 29, wherein the antifreeze is methyl
ethylene glycol.

31. The method as claimed in claim 30, wherein the antifreeze is injected
before the compressor unit is started.

32. The method as claimed in claim 31, wherein the antifreeze is added
only before the compressor unit is started.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is the US National Stage of International
Application No. PCT/EP2007/052755, filed Mar. 22, 2007 and claims the
benefit thereof. The International Application claims the benefits of
european application No. 06006071.2 filed Mar. 24, 2006, both of the
applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

[0002]The invention relates to a method for operation of a compressor
unit, in particular for underwater operation. The invention also relates
to a compressor unit, in particular for underwater operation, comprising
a compressor and an electric motor, which compressor unit has a housing
with an inlet and an outlet for the pumping medium, with a rotation axis
about which a rotor of the compressor unit can rotate.

BACKGROUND OF THE INVENTION

[0003]Recent developments in the field of compressor design have also been
concentrated on undersea arrangements of large compressors which are
intended to be used for the pumping of natural gases.

[0004]Because of the particular operating conditions, in particular
because of the greatly restricted accessibility both for maintenance
purposes and by means of supply lines, the specialists are confronted
with major requirements. The relevant environmental regulations forbid
any exchange of substances between the equipment to be installed and the
surrounding sea water. Furthermore, sea water is an aggressive medium and
extreme pressure and temperature conditions can be found at the various
depths in the sea. A further requirement is that the equipment should on
the one hand have an extremely long life and on the other hand must be
designed to be virtually free of maintenance. An additional exacerbating
factor is not-inconsiderable contamination of the medium to be pumped
which in some cases is chemically aggressive.

[0005]A compressor unit of the abovementioned type has already been
disclosed in international patent application WO 02/099286 A1. With the
aim of simplification, without any compromises, in order to reduce the
maintenance effort, and of achieving a long life at the same time, this
document proposes that the compressor rotor be formed integrally with the
motor rotor and be mounted at each of the ends by means of just two
radial bearings.

[0006]In addition, it is known from European patent application EP 1 074
746 B1 for a turbocompressor to be equipped with three radial bearings,
with the motor rotor being connected to the compressor rotor by means of
a coupling.

[0007]WO 2005/003512 A1 has already disclosed a compressor unit for
under-sea compression, to which an automation unit is connected by means
of special connectors which are suitable for under-sea use. In addition
GB 370 003 A discloses the injection of an antifreeze during the
compression of air.

[0008]The compression of fluids close to the freezing point may be
problematic. When natural gas is being pumped, the development relating
to the formation of gas hydrates results in considerable problems. Gas
hydrates are inclusion compounds which are similar to ice and in which
small gas molecules, for example noble gases and various natural gas
components, are surrounded in a cage of water molecules. Hydrate
formation must be expected even with small amounts of liquid water and at
temperatures of, for example, 10° C. The major gas catastrophe in
the year 1988 on the Norwegian North Sea drilling rig Piper Alpha was
supposedly due to such hydrate formation. Considerable additional
operation costs are also incurred in natural gas pumping as a result of
gas hydrate deposits, since they are deposited in pipelines, blocking
them.

SUMMARY OF INVENTION

[0009]The invention is based on the object of providing a method for
operation of a compressor, and a compressor unit, which very largely
minimizes the risk of gas hydrate formation, for example when pumping
natural gas under the sea.

[0010]The invention solves the problem by proposing a method for operation
of a compressor unit, and a compressor unit as recited in the claims. The
dependent claims, which respectively refer back to them, contain
advantageous developments of the invention.

[0011]One particular advantage of the invention is the reliable protection
against hydrate formation, as a result of the injection of the
antifreeze. This not only allows protection of susceptible components of
the compression unit but also of the entire pumping path, starting from
the point at which the pumping medium is injected to the subsequent
separation point. The method is also particularly advantageous because
separation of undesirable additives is carried out in any case during the
course of the chemical treatment of natural gases in a base station which
is adjacent to the compressor unit after a pipeline. The resultant
operational reliability is expressed both in higher availability of the
compressor and in a high degree of safety against blocking hydrate
formation in the pipeline which is connected to the compressor unit.

[0012]The antifreeze can be injected in the intake connecting stub, or
directly in the compressor. Application of antifreeze to components of
the compressor unit is particularly expedient for the bearings, the
electric motor and other moving parts. If there is a particular risk of
hydrate formation in the overflow area of individual compressor stages,
antifreeze can also expediently be injected here. The primary field of
application of the invention is the pumping of natural gas, since the
risk of the formation of gas hydrates is relatively high here.

[0013]In particular, various alcohols make it possible to ensure
protection against freezing of the gases. The injection of methyl
ethylene glycol is worthwhile both for financial and technical reasons.

[0014]A somewhat more economic variant of obtaining safety against hydrate
formation is to inject antifreeze at the critical points in the
compressor unit before the compressor unit is started, in particular at
the points mentioned above. One advantageous development of the invention
provides that an amount of antifreeze is injected at the sensitive points
in the compressor unit before each planned stop of the machine.

[0015]It is particularly expedient to use the antifreeze both before each
start and before each machine stop. In the case of emergency stopping or
tripping of the compressor unit, the primary factor of interest is to
stop the machine as quickly as possible, so that it may generally not be
possible to previously inject the antifreeze. Another possibility is to
cause the antifreeze to be injected at the same time that the machine
stop is initiated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]The invention will be described in more detail in the following text
using one specific exemplary embodiment and with reference to the
drawings. The illustrated embodiment should be regarded only as an
illustration, as an example of the invention. In the FIGURE:

[0017]FIG. 1 shows a schematic illustration of a longitudinal section
through a compressor unit according to the invention and the major
adjacent modules, which is operated using the method according to the
invention.

DETAILED DESCRIPTION OF INVENTION

[0018]FIG. 1 shows, schematically, a section along a compressor unit 1
according to the invention which has, as major components, a motor 2 and
a compressor 3 in a gas-tight housing 4. The housing 4 accommodates the
motor 2 and the compressor 3. The housing 4 is provided with an inlet 6
and an outlet 7 in the area of the junction between the motor 2 and the
compressor 3, with the fluid to be compressed being sucked in through the
inlet 6 by means of a suction connecting stub 8, and with the compressed
fluid flowing out through the outlet 7.

[0019]The compressor unit 1 is arranged vertically during operation, with
a motor rotor of the motor 2 above a compressor rotor 9 of the compressor
3 being combined to form a common shaft 19 which rotates about a common
vertical rotation axis 60.

[0020]The motor rotor is borne in a first radial bearing 21 at the upper
end of the motor rotor.

[0021]The compressor rotor 9 is borne by means of a second radial bearing
22 in the lower position.

[0022]An axial bearing 25 is provided at the upper end of the common shaft
19, that is to say at the upper end of the motor rotor. The radial
bearings and the axial bearing operate electromagnetically and are each
encapsulated. In this case, the radial bearings extend around the
respective bearing point of the shaft 19 in the circumferential direction
and in this case are circumferential through 360° and are
undivided.

[0023]The compressor 3 is in the form of a centrifugal compressor and has
three compressor stages 11 which are each connected by means of an
overflow 33. The pressure differences which result across the compressor
stages 11 ensure that there is a thrust on the compressor rotor 9 which
is transmitted on the motor rotor and is directed against the force of
gravity from the entire resultant rotor comprising the compressor rotor 9
and the motor rotor, thus resulting in a very high degree of thrust
matching during rated operation. This allows the axial bearing 25 to be
designed to be comparatively smaller than if the rotation axis 60 were to
be arranged horizontally.

[0024]The electromagnetic bearings 21, 22, 25 are cooled to the operating
temperature by means of a cooling system (not illustrated in detail),
with the cooling system providing a tap in an overflow 33 of the
compressor 3. A portion of the pumping medium, which is preferably
natural gas, is passed from the tap by means of pipelines through a
filter, and is then passed through two separate pipelines to the
respective outer bearing points (first radial bearing 21 and second
radial bearing 22 as well as the axial bearing 25). This cooling by means
of the cold pumping medium 80 saves additional supply lines.

[0025]The motor rotor is surrounded by a stator 16 which has encapsulation
such that the aggressive pumping medium 80 does not damage the windings
of the stator 16. The encapsulation is in this case preferably designed
such that it can contribute to the full operating pressure. This is also
because a separate cooling arrangement is provided for the stator, in
which cooling arrangement a dedicated cooling medium circulates.

[0026]The compressor rotor 9 expediently has a compressor shaft 10 on
which the individual compressor stages 11 are mounted. This can
preferably be done by means of a thermal shrink fit. An interlock, for
example by means of polygons, is likewise possible. Another embodiment
provides for different compressor stages 11 to be welded to one another,
thus resulting in an integral compressor rotor 9.

[0027]The pumping medium 80 or natural gas NG is passed from the natural
reservoir first of all into a condensate separator 81, which separates
condensates 82, including water, from the gaseous phase. The condensates
82 are passed into a condensate line 84, into which a downstream drain
line 95 also opens, which introduces condensates that have been deposited
in the compressor unit into the condensate line 84. The condensates 84
are passed from a condensate pump 85 to a mixing unit 86, in which they
are mixed with the compressed natural gas NG or pumping medium 80. The
resultant mixture is pumped into a pipeline 87 in the direction of a base
station 89.

[0028]The compressor unit 1 has a system for distribution of antifreeze
73, comprising distribution lines 94 and injection modules 72. The
antifreeze 73 is pumped from a reservoir tank 92 by means of a metering
pump 93 to the various injection modules 72 on the compressor unit 1. The
injection modules 72 locally apply antifreeze to the first radial bearing
21, to the axial bearing 25, to the second radial bearing 22 and to the
overflows 33. A further injection module 72 is located on the intake
connecting stub 8, by means of which module the antifreeze 73 is injected
directly into the pumping medium 80 which is sucked in.

[0029]Part of the injected antifreeze 73 is deposited in the compressor
unit 1, to be precise such that it is emitted through a drain 96 (at the
"single drain point") of the compressor unit 1 into the drain line 95.
The rest is pumped together with the compressed natural gas NG through
the outlet 7 into the mixing unit 86. The antifreeze 73, the natural gas
NG and the condensate 82 are pumped to the base station 89 at the earth's
surface through the pipeline 87. Hydrate formation in the pipeline 87 is
precluded because of the antifreeze 73 being carried with it. Before
reaching the base station 89, a further condensate separator 88 ensures
that the natural gas NG is dry, with the condensate including the
antifreeze 73 being passed to a conditioner 90 in which the antifreeze 73
is separated from the rest of the condensate 82. The conditioned
antifreeze 73 is passed back by means of a return line 91 along the
pipeline 87 to the reservoir tank 92. The closed circuit of the
antifreeze 73 ensures protection against hydrate formation on the one
hand, and on the other hand compliance with the relevant environmental